I have a huge anal fetish and I like to be humiliated. My fantasy is my boyfriend stretching my ass so much he doesn't even want to fuck it anymore.
When this happens he brings another girl in to fuck. They make me watch and tell me how nice and tight her asshole is. They tell me my loose ass could take anything. Then after coming they would shove random objects in my ass to proove it.

I have never heard in the annals of human physiology, of someone's "ass" stretched
and remain that way as if was made of some kind of material. You should contact some kind of medical media and report the phenomenon.